The invention is directed to a hearing aid circuit having an output stage which is connectable via an earphone and a battery to a circuit that comprises a variable limiting means for limiting (clipping) the maximum, acoustic output level of the earphone
In known hearing aid circuits of the type initially cited, the electronic limiting means--also referred to as "Peak Clipping=PC"--is designed as a mechanical control. The control acts as a variable, two-pole resistor in the circuit of the output stage between the battery and the earphone.
Programmable, electronic control resistors are also known. The control is usually formed with unipolar transistors (FET) By contrast to a mechanical control, the electronic control cannot be set to a value of resistance of or approximately equal to zero ohms. Consequently, a voltage drop would arise at the programmable electronic control given employment as a PC control even when the limiter is switched off (corresponds to the lowest resistance of the control). Since first the power requirement of the output stage is greatest in this setting, and second the operating voltage at hearing aid circuits for, in particular, hearing aids worn on the head usually amounts to only approximately 1.3 V, the voltage drop which increases linearly with increasing current would lead to a diminution of the maximally obtainable acoustic output level of the earphone. These conditions would have an especially disadvantageous effect, particularly given employment of programmable electronic controls in power output stages, for example push-pull output stages, because of their increased power consumption.
In order to avoid this disadvantage, electronic limiting in the output stage has been abandoned in hearing aid circuits having a power output stage, and the preceding preselector stage has been equipped with a programmable electronic control as a limiting means. What is disadvantageous, however, is that such a hearing aid circuit either has an increased non-linear distortion factor in the region of linear gain, or the gain is also lowered in an undesirable way given reduction of the maximum output level even by a few dB.
In hearing aid circuits having a mechanical control (potentiometer) as a limiting means, the control must be arranged at an outside wall of the hearing aid because of the needed operability. Relatively long connecting lines thereby usually become necessary. These lines are traversed by the maximum current of the output stage in the zero ohm setting of the control. An electromagnetic field is thus generated that can lead to undesired coupling to a telephone coil present in the hearing aid which, as intended, should respond to electromagnetic fields of a telephone.